Stents are generally cylindrical shaped devices that are radially expandable to hold open a segment of a vessel or other anatomical lumen after implantation into the body lumen. Stents have been developed with coatings to deliver drugs or other therapeutic agents.
Various types of stents are in use, including expandable and self-expanding stents. Expandable stents generally are conveyed to the area to be treated on balloon catheters or other expandable devices. For insertion, the stent is positioned in a compressed configuration along the delivery device, for example crimped onto a balloon that is folded or otherwise wrapped about a guide wire that is part of the delivery device. After the stent is positioned across the lesion, it is expanded by the delivery device, causing the length of the stent to contract and the diameter to expand. For a self-expanding stent, commonly a sheath is retracted, allowing expansion of the stent.
Stents are used in conjunction with balloon catheters in a variety of medical therapeutic applications including intravascular angioplasty. For example, a balloon catheter device is inflated during PTCA (percutaneous transluminal coronary angioplasty) to dilate a stenotic blood vessel. The stenosis may be the result of a lesion such as a plaque or thrombus. After inflation, the pressurized balloon exerts a compressive force on the lesion, thereby increasing the inner diameter of the affected vessel. The increased interior vessel diameter facilitates improved blood flow. Soon after the procedure, however, a significant proportion of treated vessels re-narrow.
To prevent restenosis, short flexible cylinders, or stents, constructed of metal or various polymers, are implanted within the vessel to maintain lumen size. The stents acts as a scaffold to support the lumen in an open position. Various configurations of stents include a cylindrical tube defined by a mesh, interconnected stents or like segments. Some exemplary stents are disclosed in U.S. Pat. No. 5,292,331 to Boneau, U.S. Pat. No. 6,090,127 to Globerman, U.S. Pat. No. 5,133,732 to Wiktor, U.S. Pat. No. 4,739,762 to Palmaz and U.S. Pat. No. 5,421,955 to Lau.
Stent insertion, however, may cause undesirable reactions such as inflammation, infection, thrombosis, and proliferation of cell growth that occludes the passageway. One approach to minimize these undesirable reactions has been to provide the stents with an ability to deliver one or more therapeutic agents. Coated stents, for example, may have coatings to deliver drugs or other therapeutic agents at the site of the stent that may assist in preventing these conditions. The coatings must be bioengineered to control the release of highly potent and potentially toxic drugs. Besides coating the stent to provide the stents with an ability to deliver one or more therapeutic agents, other approaches may also be used, such as sheath wrapped stents (such as those depicted in Froix, U.S. Pat. No. 6,019,789) as well as other delivery matrices (such as those depicted in Kaplan, U.S. Pat. No. 5,342,348).
When extending an area of treatment beyond the length of an individual coated stent, a second coated stent may be positioned at least partially within a previously delivered coated stent, resulting in an overlap of the coatings. Assuming each coating is used to deliver a therapeutic agent, then the area of overlap may receive a double dose of the therapeutic agent. This over dosing may cause undesirable side effects to the artery and potentially grave consequences for the patient. Therefore, it would be desirable to have a stent system which delivers one or more therapeutic agents through coatings or other approaches but which avoids the possibility of overdosing in the area of stent overlap.